Molecular cloning and functional analysis of pea cDNA E86 encoding homologous protein to hypersensitivity-related hsr203J

Comprehensive analysis of the carboxylesterase gene reveals that NtCXE22 regulates axillary bud growth through strigolactone metabolism in tobacco

Carboxylesterases (CXE) are a class of hydrolytic enzymes with α/β-folding domains that play a vital role in plant growth, development, stress response, and activation of herbicide-active substances. In this study, 49 Nicotiana tabacum L. CXE genes (NtCXEs) were identified using a sequence homology search. The basic characteristics, phylogenetic evolution, gene structure, subcellular location, promoter cis-elements, and gene expression patterns of the CXE family were systematically analyzed. RNA-seq data and quantitative real-time PCR showed that the expression level of CXEs was associated with various stressors and hormones; gene expression levels were significantly different among the eight tissues examined and at different developmental periods. As a new class of hormones, strigolactones (SLs) are released from the roots of plants and can control the germination of axillary buds.NtCXE7, NtCXE9, NtCXE22, and NtCXE24 were homologous to Arabidopsis SLs hydrolase AtCXE15, and changes in their expression levels were induced by topping and by GR24 (a synthetic analogue of strigolactone). Further examination revealed that NtCXE22-mutant (ntcxe22) plants generated by CRISPR-Cas9 technology had shorter bud outgrowth with lower SLs content. Validation of NtCXE22 was also performed in NtCCD8-OE plants (with fewer axillary buds) and in ntccd8 mutant plants (with more axillary buds). The results suggest that NtCXE22 may act as an efficient SLs hydrolase and affects axillary bud development, thereby providing a feasible method for manipulating endogenous SLs in crops and ornamental plants.

Function and application of a non-ester-hydrolyzing carboxylesterase discovered in tulip

Plants have evolved secondary metabolite biosynthetic pathways of immense rich diversity. The genes encoding enzymes for secondary metabolite biosynthesis have evolved through gene duplication followed by neofunctionalization, thereby generating functional diversity. Emerging evidence demonstrates that some of those enzymes catalyze reactions entirely different from those usually catalyzed by other members of the same family; e.g. transacylation catalyzed by an enzyme similar to a hydrolytic enzyme. Tuliposide-converting enzyme (TCE), which we recently discovered from tulip, catalyzes the conversion of major defensive secondary metabolites, tuliposides, to antimicrobial tulipalins. The TCEs belong to the carboxylesterase family in the α/β-hydrolase fold superfamily, and specifically catalyze intramolecular transesterification, but not hydrolysis. This non-ester-hydrolyzing carboxylesterase is an example of an enzyme showing catalytic properties that are unpredictable from its primary structure. This review describes the biochemical and physiological aspects of tulipalin biogenesis, and the diverse functions of plant carboxylesterases in the α/β-hydrolase fold superfamily.

Constitutive expression of a fungus-inducible carboxylesterase improves disease resistance in transgenic pepper plants

Resistance against anthracnose fungi was enhanced in transgenic pepper plants that accumulated high levels of a carboxylesterase, PepEST in anthracnose-susceptible fruits, with a concurrent induction of antioxidant enzymes and SA-dependent PR proteins. A pepper esterase gene (PepEST) is highly expressed during the incompatible interaction between ripe fruits of pepper (Capsicum annuum L.) and a hemibiotrophic anthracnose fungus (Colletotrichum gloeosporioides). In this study, we found that exogenous application of recombinant PepEST protein on the surface of the unripe pepper fruits led to a potentiated state for disease resistance in the fruits, including generation of hydrogen peroxide and expression of pathogenesis-related (PR) genes that encode mostly small proteins with antimicrobial activity. To elucidate the role of PepEST in plant defense, we further developed transgenic pepper plants overexpressing PepEST under the control of CaMV 35S promoter. Molecular analysis confirmed the establishment of three independent transgenic lines carrying single copy of transgenes. The level of PepEST protein was estimated to be approximately 0.002 % of total soluble protein in transgenic fruits. In response to the anthracnose fungus, the transgenic fruits displayed higher expression of PR genes, PR3, PR5, PR10, and PepThi, than non-transgenic control fruits did. Moreover, immunolocalization results showed concurrent localization of ascorbate peroxidase (APX) and PR3 proteins, along with the PepEST protein, in the infected region of transgenic fruits. Disease rate analysis revealed significantly low occurrence of anthracnose disease in the transgenic fruits, approximately 30 % of that in non-transgenic fruits. Furthermore, the transgenic plants also exhibited resistance against C. acutatum and C. coccodes. Collectively, our results suggest that overexpression of PepEST in pepper confers enhanced resistance against the anthracnose fungi by activating the defense signaling pathways.

Biochemical characterisation of MdCXE1, a carboxylesterase from apple that is expressed during fruit ripening

Esters are an important component of apple (Malus×domestica) flavour. Their biosynthesis increases in response to the ripening hormone ethylene, but their metabolism by carboxylesterases (CXEs) is poorly understood. We have identified 16 members of the CXE multigene family from the commercial apple cultivar, 'Royal Gala', that contain all the conserved features associated with CXE members of the α/β hydrolase fold superfamily. The expression of two genes, MdCXE1 and MdCXE16 was characterised in an apple fruit development series and in a transgenic line of 'Royal Gala' (AO3) that is unable to synthesise ethylene in fruit. In wild-type MdCXE1 is expressed at low levels during early stages of fruit development, rising to a peak of expression in apple fruit at harvest maturity. It is not significantly up-regulated by ethylene in the skin of AO3 fruit. MdCXE16 is expressed constitutively in wild-type throughout fruit development, and is up-regulated by ethylene in skin of AO3 fruit. Semi-purified recombinant MdCXE1 was able to hydrolyse a range of 4-methyl umbelliferyl ester substrates that included those containing acyl moieties that are found in esters produced by apple fruit. Kinetic characterisation of MdCXE1 revealed that the enzyme could be inhibited by organophosphates and that its ability to hydrolyse esters showed increasing affinity (K(m)) but decreasing turnover (k(cat)) as substrate acyl carbon length increases from C2 to C16. Our results suggest that MdCXE1 may have an impact on apple flavour through its ability to hydrolyse relevant flavour esters in ripe apple fruit.

Lipid deacylating enzymes in plants: old activities, new genes

Because lipids are major components of cellular membranes, their degradation under stress conditions compromises compartmentalization. However, in addition to having structural roles, membrane lipids are also implicated in signalling processes involving the activity of lipolytic enzymes. Phospholipases D and C, acting on the polar heads of phospholipids, have been relatively well characterized in plants. In contrast, knowledge of lipid deacylating enzymes remains limited. Lipid acyl hydrolases (LAH) are able to hydrolyse both fatty acid moieties of polar lipids. They differ from phospholipases A(1) or A(2) (PLA) acting on sn-1 or sn-2 positions of phospholipids, respectively, as well as from lipases which de-esterify triacylglycerols. The free polyunsaturated fatty acids generated by deacylating enzymes can be used in the biosynthesis of oxylipins and the lysophospholipids, provided by PLAs, are also bioactive molecules. In the four decades that have passed since the first description of LAH activities in plants some enzymes have been purified. In recent years, the widespread use of molecular approaches together with the attention paid to lipid signalling has contributed to a renewed interest in LAH and has led to the identification of different gene families and the characterization of new enzymes. Additionally, several proteins with putative lipase/esterase signatures have been identified. In the present paper we review currently available data on LAHs, PLAs, triacylglycerol lipases and other putative deacylating enzymes. The roles of lipid deacylating enzymes in plant growth, development and stress responses are discussed in the context of their involvement in membrane deterioration, lipid turnover and cellular signalling.

Activation of the salicylic acid signaling pathway enhances Clover yellow vein virus virulence in susceptible pea cultivars

The wild-type strain (Cl-WT) of Clover yellow vein virus (ClYVV) systemically induces cell death in pea cv. Plant introduction (PI) 118501 but not in PI 226564. A single incompletely dominant gene, Cyn1, controls systemic cell death in PI 118501. Here, we show that activation of the salicylic acid (SA) signaling pathway enhances ClYVV virulence in susceptible pea cultivars. The kinetics of virus accumulation was not significantly different between PI 118501 (Cyn1) and PI 226564 (cyn1); however, the SA-responsive chitinase gene (SA-CHI) and the hypersensitive response (HR)-related gene homologous to tobacco HSR203J were induced only in PI 118501 (Cyn1). Two mutant viruses with mutations in P1/HCPro, which is an RNA-silencing suppressor, reduced the ability to induce cell death and SA-CHI expression. The application of SA and of its analog benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH) partially complemented the reduced virulence of mutant viruses. These results suggest that high activation of the SA signaling pathway is required for ClYVV virulence. Interestingly, BTH could enhance Cl-WT symptoms in PI 226564 (cyn1). However, it could not enhance symptoms induced by White clover mosaic virus and Bean yellow mosaic virus. Our report suggests that the SA signaling pathway has opposing functions in compatible interactions, depending on the virus-host combination.

Inactive methyl indole-3-acetic acid ester can be hydrolyzed and activated by several esterases belonging to the AtMES esterase family of Arabidopsis

The plant hormone auxin (indole-3-acetic acid [IAA]) is found both free and conjugated to a variety of carbohydrates, amino acids, and peptides. We have recently shown that IAA could be converted to its methyl ester (MeIAA) by the Arabidopsis (Arabidopsis thaliana) enzyme IAA carboxyl methyltransferase 1. However, the presence and function of MeIAA in vivo remains unclear. Recently, it has been shown that the tobacco (Nicotiana tabacum) protein SABP2 (salicylic acid binding protein 2) hydrolyzes methyl salicylate to salicylic acid. There are 20 homologs of SABP2 in the genome of Arabidopsis, which we have named AtMES (for methyl esterases). We tested 15 of the proteins encoded by these genes in biochemical assays with various substrates and identified several candidate MeIAA esterases that could hydrolyze MeIAA. MeIAA, like IAA, exerts inhibitory activity on the growth of wild-type roots when applied exogenously. However, the roots of Arabidopsis plants carrying T-DNA insertions in the putative MeIAA esterase gene AtMES17 (At3g10870) displayed significantly decreased sensitivity to MeIAA compared with wild-type roots while remaining as sensitive to free IAA as wild-type roots. Incubating seedlings in the presence of [(14)C]MeIAA for 30 min revealed that mes17 mutants hydrolyzed only 40% of the [(14)C]MeIAA taken up by plants, whereas wild-type plants hydrolyzed 100% of absorbed [(14)C]MeIAA. Roots of Arabidopsis plants overexpressing AtMES17 showed increased sensitivity to MeIAA but not to IAA. Additionally, mes17 plants have longer hypocotyls and display increased expression of the auxin-responsive DR5:beta-glucuronidase reporter gene, suggesting a perturbation in IAA homeostasis and/or transport. mes17-1/axr1-3 double mutant plants have the same phenotype as axr1-3, suggesting MES17 acts upstream of AXR1. The protein encoded by AtMES17 had a K(m) value of 13 microm and a K(cat) value of 0.18 s(-1) for MeIAA. AtMES17 was expressed at the highest levels in shoot apex, stem, and root of Arabidopsis. Our results demonstrate that MeIAA is an inactive form of IAA, and the manifestations of MeIAA in vivo activity are due to the action of free IAA that is generated from MeIAA upon hydrolysis by one or more plant esterases.

Molecular and biochemical characterization of 2-hydroxyisoflavanone dehydratase. Involvement of carboxylesterase-like proteins in leguminous isoflavone biosynthesis

Isoflavonoids are ecophysiologically active secondary metabolites of the Leguminosae and known for health-promoting phytoestrogenic functions. Isoflavones are synthesized by 1,2-elimination of water from 2-hydroxyisoflavanones, the first intermediate with the isoflavonoid skeleton, but details of this dehydration have been unclear. We screened the extracts of repeatedly fractionated Escherichia coli expressing a Glycyrrhiza echinata cDNA library for the activity to convert a radiolabeled precursor into formononetin (7-hydroxy-4'-methoxyisoflavone), and a clone of 2-hydroxyisoflavanone dehydratase (HID) was isolated. Another HID cDNA was cloned from soybean (Glycine max), based on the sequence information in its expressed sequence tag library. Kinetic studies revealed that G. echinata HID is specific to 2,7-dihydroxy-4'-methoxyisoflavanone, while soybean HID has broader specificity to both 4'-hydroxylated and 4'-methoxylated 2-hydroxyisoflavanones, reflecting the structures of isoflavones contained in each plant species. Strikingly, HID proteins were members of a large carboxylesterase family, of which plant proteins form a monophyletic group and some are assigned defensive functions with no intrinsic catalytic activities identified. Site-directed mutagenesis with soybean HID protein suggested that the characteristic oxyanion hole and catalytic triad are essential for the dehydratase as well as the faint esterase activities. The findings, to our knowledge, represent a new example of recruitment of enzymes of primary metabolism during the molecular evolution of plant secondary metabolism.

The carboxylesterase gene family from Arabidopsis thaliana

Carboxylesterases hydrolyze esters of short-chain fatty acids and have roles in animals ranging from signal transduction to xenobiotic detoxification. In plants, however, little is known of their roles. We have systematically mined the genome from the model plant Arabidopsis thaliana for carboxylesterase genes and studied their distribution in the genome and expression profile across a range of tissues. Twenty carboxylesterase genes (AtCXE) were identified. The AtCXE family shares conserved sequence motifs and secondary structure characteristics with carboxylesterases and other members of the larger alpha/beta hydrolase fold superfamily of enzymes. Phylogenetic analysis of the AtCXE genes together with other plant carboxylesterases distinguishes seven distinct clades, with an Arabidopsis thaliana gene represented in six of the seven clades. The AtCXE genes are widely distributed across the genome (present in four of five chromosomes), with the exception of three clusters of tandemly duplicated genes. Of the interchromosomal duplication events, two have been mediated through newly identified partial chromosomal duplication events that also include other genes surrounding the AtCXE loci. Eighteen of the 20 AtCXE genes are expressed over a broad range of tissues, while the remaining 2 (unrelated) genes are expressed only in the flowers and siliques. Finally, hypotheses for the functional roles of the AtCXE family members are presented based on the phylogenetic relationships with other plant carboxylesterases of known function, their expression profile, and knowledge of likely esterase substrates found in plants.

cDNAs generated from individual epidermal cells reveal that differential gene expression predicting subsequent resistance or susceptibility to rust fungal infection occurs prior to the fungus entering the cell lumen

As the cowpea rust fungus penetrates the wall of a cowpea epidermal cell, resistant and susceptible plants exhibit different ultrastructural and cytochemical changes within the epidermal protoplast. To examine plant gene expression at this stage of infection, cytoplasm was extracted from individual inoculated or uninoculated epidermal cells before the fungal penetration peg reached the cell lumen. Initial differential colony hybridization screening of an expressed sequence tag library constructed from globally amplified cDNAs generated from the inoculated resistant cells resulted in 80 clones (out of 835) with a differential hybridization pattern. Further slot-blot screening and screening of the amplified cDNAs generated from inoculated or uninoculated, resistant or susceptible cells revealed 28 separate genes, mostly with no matching sequences in the databases, that were up-regulated in response to the growth of the fungus through the wall of resistant or susceptible cells. Five genes, including those coding for beta- and alpha-tubulin, were found to be down-regulated specifically in inoculated, susceptible cells, and five were specifically up-regulated in inoculated, resistant cells, including a PR-10 homolog and a phenylalanine ammonia-lyase gene. Probing the amplified cDNAs from each cell type for the expression of cell death-related genes revealed that an LLS1 homolog (vuLLS1), cloned from cowpea, was up-regulated by infection in both resistant and susceptible cells and that a homolog of HSR203J was differentially up-regulated in resistant cells. These data show that changes in gene expression predicting the subsequent expression of susceptibility or hypersensitive resistance to fungal infection occur prior to the fungus entering the cell lumen.

Expression of the 12-oxophytodienoic acid 10,11-reductase gene in the compatible interaction between pea and fungal pathogen

Suppressors produced by Mycosphaerella pinodes are glycopeptides to block pea defense responses induced by elicitors. A clone, S64, was isolated as cDNA for suppressor-inducible gene from pea epicotyls. The treatment of pea epicotyls with suppressor alone induced an increase of S64 mRNA within 1 h, and it reached a maximum level at 3 h after treatment. The induction was not affected by application of the elicitor, indicating that the suppressor has a dominant action to regulate S64 gene expression. S64 was also induced by inoculation with a virulent pathogen, M. pinodes, but not by inoculation with a non-pathogen, Ascochyta rabiei, nor by treatment with fungal elicitor. The deduced structure of S64 showed high homology to 12-oxophytodienoic acid reductase (OPR) in Arabidopsis thaliana. A recombinant protein derived from S64 had OPR activity, suggesting compatibility-specific activation of the octadecanoid pathway in plants. Treatment with jasmonic acid (JA) or methyl jasmonic acid, end products of the octadecanoid pathway, inhibited the elicitor-induced accumulation of PAL mRNA in pea. These results indicate that the suppressor-induced S64 gene expression leads to the production of JA or related compounds, which might contribute to the establishment of compatibility by inhibiting the phenylpropanoid biosynthetic pathway.